1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to a fiber amplifier for amplifying C-band and L-band optical signals propagating in the optical transmission system.
2. Description of the Related Art
To cope with the recent increase in the quantity of data transmission, a demand for a wavelength division multiplexing (WDM) optical transmission systems using an extended transmission bandwidth has developed. Accordingly, research is being actively conducted into a wide-band system using both a wavelength range of 1,530 to 1,560 nm the so-called C-band, and a wavelength range of 1,570 to 1,600 nm the so-called L-band. A n optical fiber amplifier doped with a rare-earth element, for example, erbium, is widely used to amplify optical signals in an optical transmission system. However, such erbium-doped fiber amplifiers (EDFA) have an available bandwidth limited to about 30 nm for each of C and L-bands. Although, Raman fiber amplifiers have a wide available bandwidth enabling amplification of both the C-band and the L-band, they require a high pumping power to obtain the desired gain. For this reason, EDFAs are widely used for wide-band fiber amplifiers. However, most EDFAs have a parallel arrangement for individually amplifying C-band and L-band optical signals.
FIG. 1 is a diagram illustrating of a conventional wide-band fiber amplifier. As shown in FIG. 1, the conventional wide-band fiber amplifier designated by the reference numeral 100 is connected to an external optical fiber 110. Wide-band fiber amplifier 100 includes first and second amplifying units 160 and 170, and first and second wavelength selective couplers 121 and 122 (WSC1 and WSC2), adapted to couple first and second amplifying units 160 and 170 in parallel.
First wavelength selective coupler 121 splits an optical signal with wavelengths of 1,550 and 1,590 nm into components having wavelengths of 1,550 nm (C-band) and 1,590 nm (L-band), respectively. T hereafter, it outputs the C-band optical signal to first amplifying unit 160 and the L-band optical signal to second amplifying unit 170.
First amplifying unit 160 includes first and second optical isolators 131 and 132 (ISO1 and IS02), first and second pump laser diodes 141 and 142 (PUMP LD1 and PUMP LD2), third and fourth wavelength selective couplers 123 and 124 (WSC3 and WSC4), and a first erbium-doped fiber 151 (EDF1). Each of first and second optical isolators 131 and 132 serves to cut off light from first erbium-doped fiber 151 advancing backwards, such as amplified spontaneous emission (ASE) noise or reflected light. First pump laser diode 132 outputs a first pumping light with a wavelength of 980 nm. Third wavelength selective coupler 123 outputs the first pumping light and C-band optical signal, inputted thereto, to first erbium-doped fiber 151. Second pump laser diode 142 outputs a second pumping light with a wavelength of 1,480 nm. Fourth wavelength selective coupler 124 outputs the second pumping light, inputted thereto, to first erbium-doped fiber 151, while passing the amplified C-band optical signal inputted thereto. First erbium-doped optical fiber 151 is bidirectionally pumped by the first and second pumping light, thereby amplifying the C-band optical signal inputted thereto.
Second amplifying unit 170 includes third and fourth optical isolators 133 and 134 (IS03 and IS04), third and fourth pump laser diodes 143 and 144 (PUMP LD3 and PUMP LD4), fifth and sixth wavelength selective couplers 125 and 126 (WSC5 and WSC6), and a second erbium-doped fiber 152 (EDF2). Each of third and fourth optical isolators 133 and 134 serves to cut off light from second erbium-doped fiber 152 advancing backwards, such as ASE noise or reflected light. Third pump laser diode 143 outputs a third pumping light with a wavelength of 980 nm. Fifth wavelength selective coupler 125 outputs the third pumping light and L-band optical signal, inputted thereto, to second erbium-doped fiber 152. Fourth pump laser diode 144 outputs a fourth pumping light with a wavelength of 1,480 nm. Sixth wavelength selective coupler 126 outputs the fourth pumping light, inputted thereto, to second erbium-doped fiber 152, while passing the amplified L-band optical signal inputted thereto. Second erbium-doped optical fiber 152 is bidirectionally pumped by the first and second pumping light, thereby amplifying the L-band optical signal inputted thereto.
Second wavelength selective coupler 122 couples the C-band and L-band optical signals respectively inputted thereto from first and second amplifying units 160 and 170, and outputs the resultant optical signal via external optical fiber 110.
However, conventional wide-band fiber amplifiers having the above mentioned configuration requires an increased length of the second erbium-doped fiber and high pumping power because the second amplifying unit has a low amplification efficiency. Furthermore, such a configuration suffers other limitations such as the wide-band fiber amplifier exhibiting a high noise figure in the L-band.